Perfusionists, formerly known as cardiovascular perfusionists or clinical perfusionists, play a crucial role in the field of cardiovascular surgery. They operate what is known as the "heart-lung machine," among other duties. The perfusionist is responsible for all aspects of the heart-lung machine whenever it becomes necessary to interrupt or replace the functioning of the heart by circulating blood outside of a patient's body. There are approximately 4,000 perfusionists employed in the United States.